I. Field of the Invention
This invention relates to a process for producing small particles of aluminum nitride. In particular, the invention produces a process capable of producing small particles of aluminum nitride suitable for producing a sintered product.
II. Description of the Prior Art
Aluminum nitride is attracting much interest nowadays for various commercial uses but particularly as a substrate for electronic components. It has been found that aluminum nitride has properties which make it particularly suitable for this purpose. In particular, pure or relatively pure aluminum nitride combines high thermal conductivity and low electrical conductivity. Accordingly, electronic substrates formed on an aluminum nitride substrate show less tendency to become overheated than components formed on other materials without any loss of reliability due to current leakage or the like.
Substrates for electronic components can be made out of aluminum nitride by sintering together suitable particles of this material. However, aluminum nitride powders currently used for such sintering procedures are not ideal because they are normally made up of particles having a wide distribution of sizes. For best performance, the powder should be made up of approximately submicron-sized particles having a relatively narrow size distribution, but conventional procedures for producing aluminum nitride do not normally produce powders of this kind and so additional grinding and classification steps are required if an ideal substrate is to be formed. These additional steps add cost and complexity to the manufacturing process.
Examples of known processes for producing aluminum nitride are briefly discussed below.
First of all, it is well known that aluminum nitride can be produced by the carbothermal reduction of alumina according to the following reaction: EQU Al.sub.2 O.sub.3 +N.sub.2 +3C.fwdarw.2AlN+3CO.
This basic reaction operates at a temperature in the range of 1400.degree. to 1800.degree. C. and tends to produce large particles having a size in the range of 1 to 10 microns. Many modifications of this basic procedure are already known, and some examples are discussed below, but none of these known processes is capable of producing an aluminum nitride powder that is ideal for sintering purposes.
Published PCT Patent Application WO-91/00842 in the name of Charles et al published on Jan. 24, 1991 relates to a process for manufacturing aluminum nitrite in which a substantially homogenous mixture of carbon and aluminum trihydroxide or its dehydration product, gamma aluminum oxide, is reacted with nitrogen by heating to a temperature of not less than 1200.degree. C. and preferably not more than 1600.degree. C. The preferred temperature is between 1450.degree. and 1550.degree. C. The particle size of the starting material is stated to be in the range of 0.5 to 1.5 microns and desirably between 0.6 and 0.8 microns. The particle size of the aluminum nitride product is substantially the same as that of the starting material. It is stated that, if the particle size is too low, there is an increase in oxygen impurities and handling problems. The resulting particles are found to be not ideal for the preparation of substrates for electronic components.
U.S. Pat. No. 4,680,278 to Inoue et al issued on Jul. 4, 1987 discloses a process for preparing aluminum nitride powder by the carbothermal reduction process mentioned above. However, the reaction is carried out in the presence of at least one additive selected from aluminum nitride powder, silicon nitride powder, silicon carbide powder and powders of certain other substances. The process is stated to be useful for preparing aluminum nitride powder of small particle size and narrow size distribution. The alumina starting material is between 1 and 20 microns in size, preferably between 2 and 5 microns. The resulting particles are often impure because of the use of the additive and it is found that the resulting particles are not ideal.
U.S. Pat. No. 4,917,877 to Oguni et al issued on Apr. 17, 1990, discloses a process for producing aluminum nitride powder by the carbothermal reduction process in which a solid organic compound is added to the mixture. The resulting particles appear to have a size of about 1 micron. Again, however, the particles are not pure and are not ideal for the intended application.
British Published Patent Application 2,127,390A to Nobuyuki Kuramoto et al published on Apr. 11, 1984 relates to a process for producing a fine powder of aluminum nitride having an average particle diameter of not more than 2 microns. The process involves firstly wet mixing an aluminum powder of less than 2 microns in size with a carbon of less than 1 micron in size and, secondly, heating the mixture to a temperature in the range of 1400.degree. to 1700.degree. C. in a nitrogen atmosphere. Again, it is found that an ideal aluminum nitride powder is not formed.